Cellulose ester having improved stability to wet heat

ABSTRACT

The present invention provides a cellulose ester in which heat resistance under wet condition is compatible with both releasability and spinnability even when the amount of the residual solvent is small in a casting process. The cellulose ester fulfills the following formula: 
 
0.5&lt;(B)/(A)&lt;1.5 
wherein (A) and (B) represent the total amount (in terms of molar amount) of residual sulfuric acid in 1 gram of the cellulose ester and the total amount (in terms of molar amount) of calcium contained in 1 gram of the cellulose ester, respectively.

TECHNICAL FIELD

The present invention relates to a cellulose ester useful for forming afilm (e.g., a polarizer protective film, a color filter, and a film of aphotosensitive material) or a fiber, and a film formed from thecellulose ester.

BACKGROUND ART

Films formed from a cellulose ester typified by a cellulose triacetateare tough and excellent in properties such as dimensional stability,heat resistance and optical isotropy. Therefore, these films have beenused in various applications such as a support (or a substrate, or abase material) of an optical film (e.g., a base film for aphotosensitive material), a polarizer protective film in Liquid crystaldisplays (LCDs) (a liquid crystal protective film) and a color filter.

Furthermore, recently in order to improve narrow viewing-angle latitudesof LCD, a technique has been proposed to produce a film which causes amoderate retardation in the thickness direction (Rt) due to a refractiveindex of the thickness direction smaller than that of a right angledirection to the thickness direction, and use the film as a retarderfilm (and a WV film). Accordingly, the cellulose ester film is requiredto satisfy optical characteristics such as a small Yellowness Index, asmall haze, a small birefringence of a film in the plane, as well as ahigh transparency. In recent years, these LCDs have been widely used asnot only display of a word processor or a personal computer but alsodisplay of a television or a Digital Versatile Disc (DVD), and theseapparatuses are required to have a wider and thinner screen.

Moreover, in accordance with enlarging a liquid crystal screen, thedemand for the wider viewing-angle latitude has been increased.Furthermore, since the portability of the LCD has been required,downsizing, particularly reducing the thickness, has been desired.

Cellulose Commun Vol5, No2 (1998) summarizes suitable physicalproperties for such a liquid crystal protective film and a WV film. Forsuch applications, it is preferred to use a film which does not havelinear drawing in molecules and have an optical isotropy in the filmplate. A cellulose ester film, or the above-mentioned liquid crystalprotective film or WV film is generally produced by casting a solution(dope). This method comprises casting a dope (solution) containing acellulose ester and a solvent on a surface-polished stainless belt ormetal drum, evaporating the organic solvent or cooling the dope forsolidification, and releasing the film before the metal supportcirculates once, and drying the film.

In the process releasing the film from the metal support, it iseffective that the release tension is controlled as low as possible onthe release for getting an objective Rt as usage. That is, in the casewhere the release tension is low, the linear drawing on the release isavoidable. Further, the good releasability from the metal support in theflow-casting enlarges the shrinkage on the metal support, and therefore,the linear drawing in the machine direction or the traverse direction isavoided.

In this way, it is necessary to shrink the film on the metal supportwhen the film is cast from a cellulose ester solution (dope). If thefilm cannot be shrinked, the orientation occurs due to the internalstress. Since the cellulose ester has a stiff chain and has bulkysubstituent(s), the once generated orientation is not disturbed easilyand exerts great influence dynamically on properties of the film. In thecase where the residual solvent is lowered, it is advantageous that thecoefficient of shrinkage is decreased. Therefore, it is desired toobtain a cellulose ester being excellent in releasability in thereleasing step of the casting in which the residual solvent is low.

In the cellulose ester film, particularly a cellulose triacetate, ispreferably used. Moreover, using a mixed fatty acid ester of cellulosethat is a cellulose esterified with a mixed fatty acid is sometimesattempted.

On the one hand, a cellulose ester is easy to be hydrolyzed. In the caseof using a cellulose ester as a protective film for a polarizer (aliquid crystal protective film), a color filter or a retarder plate inLCD which is often used for a long period, therefore, the celluloseester is required to have heat resistance under wet conditionappropriate to the envisioned long-term use. Also in this point,enlargement of LCD screen in the recent requests that the celluloseester has further improvement in heat resistance under wet condition.That is, the polarizer is damaged due to insufficient heat resistanceunder wet condition of the protective film. Further, the possibility ofdamaging is increased when the area of the cellulose ester film isbroadened.

On the other hand, concerning a cellulose diacetate, in the case ofproducing a fiber by using a dope (solution) containing a celluloseester (such as a cellulose diacetate) and a solvent, a high spinnabilityis required. A common method using a cellulose diacetate includes adry-spinning which comprises discharging a dope from a spinneret(nozzle), in which the dope contains a cellulose diacetate dissolved inan organic solvent (such as acetone), and drying the discharged matterwith evaporating the solvent by hot air or other means. In order toinhibit a clog in the spinneret (nozzle), the releasability of thecellulose acetate from a metal (metallic material) is required. Also, afiber obtained by spinning the cellulose ester is required to beenhanced in hydrolysis resistance to prevent the fiber from adhesion ofan odor of a fatty acid derived from the hydrolysis.

Regarding the cellulose diacetate, techniques for inhibiting thehydrolysis by addition of a metal salt or a metal ion have beendisclosed. Japanese Patent Application Laid-Open No. 105665/1988(JP-63-105665A) and Japanese Patent Application Laid-Open No. 96231/1989(JP-1-96231A) disclose a technique utilizing an action of a buffersolution obtained by addition of an alkali metal salt. Moreover,Japanese Patent Application Laid-Open No. 96232/1989 (JP-1-96232A)discloses a technique for inhibiting a hydrolysis of a cellulose acetatedue to the residual free acid by adding a hydroxide, for example, ahydroxide of sodium or calcium, to a dope. Further, the applicant of thepresent application discloses that in Japanese Patent ApplicationLaid-Open No. 213270/1995 (JP-7-213270A) the hydrolysis reaction can beeffectively inhibited by adding a water-soluble metal salt containing analkaline earth metal ion and/or a trivalent or higher metal ion. Theseapplications show that the more the amount of the metal ion is, thehigher the inhibition effect on the hydrolysis is. However, in the caseof increasing the amount of the metal ion, the clog or the like in thenozzle part occurs because of the releasability from the metallicmaterial on discharging the fiber. Further, in the case of film-formingfrom the cellulose ester, the releasability on flow-casting isdeteriorated, and as a result, there is a possibility that the surfacesmoothness of the obtained cellulose ester film is damaged or that sometroubles such as break occurs in reducing the thickness of the celluloseester film.

On the other hand, as a technique for improving releasability from asupport and spinnability in a solution casting process, Japanese PatentApplication Laid-Open No. 316701/1998 (JP-10-316701A) discloses atechnique that even the small amount of the residual solvent in theabove-mentioned casting process makes the releasability from the supportmore excellent when the total content of an alkali metal and an alkalineearth metal is not more than 5.5×10⁻⁶ equivalent in 1 gram of acellulose ester. The applicant of the present application discloses inJapanese Patent Application Laid-Open No. 316701/1998 (JP-10-316701A)that a cellulose acetate is capable of releasing from a support bylimiting the amount of the alkaline earth metal in the cellulose acetateeven when the amount of the residual solvent is small with the use of anexcellent casting process. In the cellulose acetate, however, heatresistance under wet condition was insufficient.

Moreover, Japanese Patent Application Laid-Open No. 131536/2002(JP-2002-131536A) discloses a composition in which the content of analkaline earth metal is 1 to 50 ppm in a polarizer protective film,which is formed from a mixed fatty acid ester of cellulose (a celluloseacetate propionate). This document mentions that the amount of residualsulfuric acid (as the content of sulfur element) in the mixed fatty acidester is 1 to 50 ppm. Further, Japanese Patent Application Laid-Open No.310640/1999 (JP-11-310640A) discloses to use a cellulose acylatecontaining 10 to 100 ppm of an alkaline earth metal in a preparationmethod of a cellulose acylate. Furthermore, Japanese Patent ApplicationLaid-Open No. 314811/2000 (JP-2000-314811A) discloses a cellulose esterfilm having a specific molecular weight distribution, and JapanesePatent Application Laid-Open No. 62430/2000 (JP-2000-62430A) disclosesan optical film which comprises a cellulose ester having a specificsubstitution degree of acyl group. These documents mention that theamount of a calcium component is not more than 60 ppm. Japanese PatentApplication Laid-Open No. 40244/2002 (JP-2002-40244A) discloses acomposition in which the content of an alkaline earth metal is not morethan 30 ppm for a film of a cellulose ester.

Moreover, Japanese Patent Application Laid-Open No. 5851/1999(JP-11-5851A) discloses that, if a cellulose ester has a specific rangein the total substitution degree of acyl groups at 2-, and 3-positionsand a specific range in the substitution degree of acyl group at6-position, a film having an excellent retardation (Rt) in the thicknessdirection can be obtained even through cooling at a low temperature.However, this document has no disclosure regarding heat resistance underwet condition or releasability. Moreover, Japanese Patent ApplicationLaid-Open No. 212338/2002 (JP-2002-212338A) discloses that a solution ofa cellulose ester in which the total substitution degree of acyl groupsat 2-, and 3-positions is in a specific range and the substitutiondegree of acyl group at 6-position is in a specific range has a lowviscosity within a practicable dope concentration range. This documentmentions an addition of a metal salt, however, a heat resistance underwet condition thereof is unsatisfactory.

As just described, in any techniques mentioned above, heat resistanceunder wet condition of a cellulose ester cannot be compatible with theboth properties of releasability from a support and spinnability evenwhen the amount of the residual solvent is small in the solution castingprocess.

It is therefore an object of the present invention to provide acellulose ester which has a high releasability from a support (that is,has a low release strength) through a casting (or a solution casting)process as well as is excellent in heat resistance under wet condition,and a dope containing the cellulose ester. It is another object of theinvention to provide a cellulose ester not only which has a highreleasability from a support in a casting process and is excellent inheat resistance under wet condition but also which is high in opticalcharacteristics, and to provide a dope containing the cellulose ester.It is still another object of the invention to provide a cellulose esterhaving a high spinnability on producing a fiber with a dope of thecellulose ester, and a dope containing the cellulose ester. It is afurther object of the invention to provide a cellulose ester film havingreleasability, optical characteristics and spinnability, and furtherbeing excellent in heat resistance under wet condition.

DISCLOSURE OF THE INVENTION

The inventors of the present invention made intensive studies to achievethe above objects and finally found that (a) regarding a celluloseester, the ratio of calcium in alkaline earth metals relative to theresidual sulfuric acid group exerts an influence on heat resistanceunder wet condition or releasability from a metallic material; that (b)among alkaline earth metals, only calcium has a significant effect onheat resistance under wet condition, and other alkaline earth metalsaffect on hydrolysis but are not very responsible for heat resistanceunder wet condition; that (c) there is a maximum limited amount forcalcium to exert a good releasability from a metallic material; and that(d) there is a minimum limited amount for calcium to exert the preferredheat resistance under wet condition; and further that (e) a celluloseacetate in which the degrees of substitution at 2- and 3-positions arein a specific range and the degree of substitution at 6-position is in aspecific range and is used in combination with the above-mentioned (a)contributes to obtain a cellulose ester film having an excellentsolution stability, an excellent releasability from a metallic material,or smoothness, and an improved heat resistance under wet condition canbe obtained even in the case of using a solvent other than methylenechloride. The present invention was accomplished based on the abovefindings.

That is, the present invention includes

(1) a cellulose ester fulfilling the following formula:0.5<(B)/(A)<1.5

wherein (A) is the total amount (in terms of molar amount) of residualsulfuric acid in 1 gram of said cellulose ester (hereinafter, sometimesreferred to as a total amount of the residual sulfuric acid), and (B) isthe total amount (in terms of molar amount) of calcium contained in 1gram of said cellulose ester;

(2) a cellulose ester fulfilling the following formula:0.5<(B)/(A)<1.2

wherein (A) is the total amount (in terms of molar amount) of residualsulfuric acid in 1 gram of said cellulose ester, and (B) is the totalamount (in terms of molar amount) of calcium contained in 1 gram of saidcellulose ester;

(3) a cellulose ester fulfilling the following formula:0.6<(B)/(A)<1.0

wherein (A) is the total amount (in terms of molar amount) of residualsulfuric acid in 1 gram of said cellulose ester, and (B) is the totalamount (in terms of molar amount) of calcium contained in 1 gram of saidcellulose ester;

(4) a cellulose ester fulfilling the following formula:0.75<(B)/(A)<1.0

wherein (A) is the total amount (in terms of molar amount) of residualsulfuric acid in 1 gram of said cellulose ester, and (B) is the totalamount (in terms of molar amount) of calcium contained in 1 gram of saidcellulose ester;

(5) a cellulose ester according to any one of the above-mentioned esters(1) to (4), wherein the total amount (A) of residual sulfuric acid in 1gram of said cellulose ester is 1×10 ⁻⁷ to 500×10⁻⁷ mol;

(6) a cellulose ester according to any one of the above-mentioned esters(1) to (5), wherein the total amount (B) (in terms of molar amount) ofcalcium contained in 1 gram of said cellulose ester fulfills thefollowing formula:5×10⁻⁷<(B)<20×10⁻⁷;(7) a cellulose ester according to any one of the above-mentioned esters(1) to (5), wherein the total amount (B) (in terms of molar amount) ofcalcium contained in 1 gram of said cellulose ester fulfills thefollowing formula:8×10⁻⁷<(B)<15×10⁻⁷;(8) a cellulose ester according to any one of the above-mentioned esters(1) to (7), which is a cellulose acetate and has an average degree ofacetylation from 43.7 to 62.5%;(9) a cellulose ester-according to any one of the above-mentioned esters(1) to (8), which is a mixed fatty acid ester of cellulose;(10) a cellulose ester according to any one of the above-mentionedesters (1) to (8), which is a cellulose acetate (a cellulosetriacetate), in which the degrees of substitution at 2- and 3-positionsare not less than 1.70 and not more than 1.95, and the degree ofsubstitution at 6-position is not less than 0.88;(11) a cellulose ester according to any one of the above-mentionedesters (1) to (8), which is a cellulose acetate (a cellulose triacetate)in which the degrees of substitution at 2- and 3-positions are not lessthan 1.80 and not more than 1.95, and the degree of substitution at6-position is not less than 0.88 and not more than 0.95;(12) a cellulose ester according to any one of the above-mentionedesters (1) to (8), which is a cellulose acetate (a cellulosetriacetate), wherein the degrees of substitution at 2- and 3-positionsare not less than 1.84 and not more than 1.92, and the degree ofsubstitution at 6-position is not less than 0.89 and not more than 0.92;(13) a cellulose ester according to any one of the above-mentionedesters (1) to (12), which is produced by using a sulfuric acid catalyst;(14) a cellulose ester according to any one of the above-mentionedesters (1) to (13), which comprises adding water and/or magnesiumacetate and/or magnesium hydroxide at the time when esterification iscompleted, and adding calcium hydroxide on and after a ripening step forstabilizing the cellulose ester;(15) an optical film comprising a cellulose acetate recited in any oneof the above-mentioned (1) to (14);(16) an optical film according to the above-mentioned (15), which is anyone of films selected from the group consisting of a protective film fora polarizer, a retarder film, a scattering film, and a widen viewingangles (or viewing angle compensator) film (a WV film);(17) a process for producing a cellulose ester recited in any one ofcellulose esters of the above-mentioned (1) to (14), which comprisesacylating a cellulose in the presence of sulfuric acid catalyst, andhydrolyzing (or deacylating) the acylated cellulose to obtain thecellulose ester; and further includes a step for neutralizing saidsulfuric acid at least in part with a neutralizing agent, and a step foradding a calcium component thereto;(18) a process according to the above-mentioned (17), which comprisesacylating (particularly, acetylating) the cellulose with an acylatingagent (particularly, an acetylating agent) in the presence of sulfuricacid catalyst, hydrolyzing (or ripening) the acetylated cellulose, andadding the calcium component thereto to obtain the cellulose ester,

wherein the molar ratio (B)/(A) determined from (A) the total amount (interms of molar amount) of residual sulfuric acid in 1 gram of thecellulose ester and (B) the total amount (in terms of molar amount) ofcalcium contained in 1 gram of said cellulose ester is adjusted into therange of the following formula:0.5<(B)/(A)<1.5by addition of the calcium component, and

the addition of the calcium component (particularly, calcium hydroxide)is carried out after said sulfuric acid is neutralized at least in partwith a neutralizing agent (particularly, a neutralizing agent containinga magnesium component) at (i) the time when the acylating (particularly,acetylating) step is finished and before the ripening step is notstarted yet, or (ii) the time when the hydrolyzing step is finished andthe adding step of the calcium component is not started yet.

(19) a method for improving heat resistance under wet condition of acellulose ester, which comprises adjusting the molar ratio (B)/(A)determined from (A) the total amount (in terms of molar amount) ofresidual sulfuric acid in 1 gram of said cellulose ester and (B) thetotal amount (in terms of molar amount) of calcium contained in 1 gramof said cellulose ester into the range of the following formula.0.5<(B)/(A)<1.5

Throughout this specification, the term “a cellulose acetate” means acellulose ester in which an acyl group as a substituent fundamentallycomprises only acetyl group, not comprises other acyl groups (e.g., anacyl group having three or more carbon atoms, such as propionyl group).

Moreover, throughout this specification, the phrase “the total amount ofresidual sulfuric acid in a cellulose ester” means not only residualfree sulfuric acid (H₂SO₄) in a cellulose ester, but also sulfuric acidgroup (sulfo group) bonded to a cellulose.

DETAILED DESCRIPTION OF THE INVENTION

The cellulose ester of the present invention can be produced byesterifying a pulp. The species of the pulp is not particularly limitedto a specific one, and may be used various pulps. Typically, at leastone pulp selected from a wood pulp (e.g., a hardwood pulp and a softwoodpulp) and a linter pulp may be used, and the wood pulp and the linterpulp may be used in combination. The content of α-cellulose, which is anindicator of purity of a pulp, may be selected within the range of, forexample, about 90 to 100% by weight, and practically about 92 to 99% ina wood pulp. According to the present invention, a low-purity pulp suchas a pulp having an α-cellulose content of about 90 to 97% (inparticular about 92 to 96%) may be used. Among these pulps, the woodpulp (e.g., the hardwood pulp) is practically used. In the presentinvention, a low-quality cellulose material (e.g., a wood pulp having ahemicellulose content of about 3 to 20% by weight (particularlypreferably about 4 to 8% by weight)) may be also used.

As described above, a cellulose ester produced by using a hardwood pulpas a raw material is generally inferior in releasability of a film inthe casting process, and a cellulose ester produced by using a softwoodpulp as a raw material is usually inferior in optical characteristics(e.g., transparency) or spinnability. The present invention can improvereleasability of a cellulose ester film and optical characteristics(such as transparency) or spinnability of a cellulose ester in spite ofusing such a wood pulp (e.g., a hardwood pulp) as a raw material.

The cellulose ester may be obtained by a conventional productiontechnology, for example, a process using sulfuric acid as a catalyst, aacetic acid process, a methylene chloride process, and others.

The cellulose ester preferably used in the present invention may bemainly classified into the following three categories:

(1) a cellulose acetate obtained by using acetic acid on esterification,

(2) a mixed fatty acid ester of cellulose, in which particularlypreferable one is obtained by esterifying a cellulose with a mixed fattyacid containing acetic acid, and

(3) a cellulose acetate (a cellulose triacetate) in which the degrees ofsubstitution at 2- and 3-positions are in a specific range and thedegree of substitution at 6-position is in a specific range.

Firstly, the embodiment (1) of the cellulose ester is described.

The cellulose ester (the cellulose acetate) may be produced by, ifnecessary, activation of a cellulose with an organic acid (e.g., aceticacid); acylation (or esterification, particularly acetylation) of thecellulose in the presence of an acid catalyst (particularly sulfuricacid); addition of water or a diluted acetic acid as a stopping step forthe esterification; if necessary, partially neutralization of the acidcatalyst; and hydrolysis of the resulting matter [that is, ripening].More specifically, the cellulose acetate may be usually produced byactivation of a pulp (cellulose) with acetic acid or the like(activating step), preparation of triacetate with acetic anhydride inthe presence of a sulfuric acid as a catalyst (acetylation step), andadjustment of the degree of acetylation by saponification (hydrolysis)(saponification or ripening step). In this method, the activating stepmay be conducted, for example, by spraying acetic acid or hydratedacetic acid to the pulp (cellulose) or dipping the pulp (cellulose) inacetic acid or hydrated acetic acid, or others. The amount of aceticacid is about 10 to 100 parts by weight, preferably about 20 to 80 partsby weight, and more preferably about 30 to 60 parts by weight, relativeto 100 parts by weight of the pulp (cellulose). The amount of aceticanhydride in the acetylation step (acetylating step, esterifying step)may be selected from a certain range so that the degree of acetylationis in the above-mentioned range. For example, the amount of aceticanhydride is about 230 to 300 parts by weight, preferably about 240 to290 parts by weight, and more preferably about 250 to 280 parts byweight, relative to 100 parts by weight of the pulp (cellulose).

In the acetylation step, as a solvent, acetic acid is usually employed.The amount of acetic acid relative to 100 parts by weight of the pulp(cellulose) is, for example, about 200 to 700 parts by weight,preferably about 300 to 600 parts by weight, and more preferably about350 to 500 parts by weight. As a catalyst for esterification step orripening step, usually sulfuric acid is employed. The amount of sulfuricacid relative to 100 parts by weight of the cellulose is usually about 1to 15 parts by weight, preferably about 5 to 15 parts by weight, andparticularly about 5 to 10 parts by weight. Moreover, the saponificationor ripening may for example be conducted at a temperature of about 50 to70° C. Incidentally, the acid catalyst (sulfuric acid) may beneutralized in part with a neutralizing agent and the partly neutralizedcatalyst is sometimes utilized as a catalyst for hydrolysis or ripening.

Incidentally, the acid catalyst (particularly sulfuric acid) is oftenneutralized by adding a neutralizing agent (or stabilizer) such as analkali or alkaline earth metal compound (e.g., a metal oxide, a metalhydroxide, and a metal salt) at a suitable step after acylation(acetylation) [for example, in a course after acetizing step beforehydrolysis; after hydrolysis or ripening; or others)] in the productionprocess of the cellulose ester (cellulose acetate). The neutralizingagent may be divisionally added in a plurality of steps. For example,after acetylation (step), the acid catalyst (sulfuric acid) may bepartially neutralized (neutralized in part) by adding the neutralizingagent, and after hydrolysis step, the acid catalyst may be furtherneutralized (completely neutralized).

In order to improve optical characteristics of a cellulose acetate,treatment of a cellulose acetate with an oxidizing agent (or an oxidant)may be conducted in a suitable stage of production, for example, afterthe acetylation, or after the saponification or ripening. Examples ofthe oxidant include hydrogen peroxide; a peracid such as performic acid,peracetic acid and perbenzoic acid; and an organic peroxide such asdiacetyl peroxide. The oxidant may be used singly or in combination. Thepreferred oxidant is one which can be easily removed from the celluloseacetate without leaving any appreciable residues, thus including, forexample, hydrogen peroxide, performic acid, and peracetic acid. Theparticularly preferred oxidant includes hydrogen peroxide or peraceticacid. The amount of the oxidant may be selected depending on the levelof the desired optical characteristics. The amount of the oxidant is,for example, about 0.01 to 5 parts by weight, preferably about 0.1 to2.5 parts by weight, and particularly about 0.1 to 1 parts by weight,relative to 100 parts by weight of the cellulose acetate. The treatmentwith the oxidant may be conducted depending on the kind of the oxidant,for example, at about 20 to 100° C. and preferably about 30 to 70° C.

The average degree of acetylation of the cellulose acetate may beselected within the range of about 30 to 62.5% in accordance withapplications or characteristics. The industrially useful celluloseacetate is usually a cellulose diacetate or a cellulose triacetate. Theaverage degree of acetylation of the cellulose acetate (cellulosediacetate or cellulose triacetate) may be usually about 43.7 to 62.5%,and preferably about 45 to 62%. More specifically, the average degree ofacetylation of the cellulose diacetate is, for example, about 43.7 to58.0% (the average degree of substitution of the acetyl group: 1.7 to2.6), preferably about 45 to 57.0% (the average degree of substitution:1.8 to 2.6), and more preferably about 48 to 57.0% (the average degreeof substitution: 2.0 to 2.6). The particularly preferred average degreeof acetylation of the cellulose diacetate is 53.0 to 56.0%.

In the case of dissolving the cellulose triacetate in a specific solvent(such as methylene chloride), the average degree of acetylation isusually about 58 to 62.5%, preferably about 58.5 to 62% and morepreferably about 59 to 62% (e.g., about 60 to 61%) for improveddimensional stability, moisture resistance, heat resistance, and others.

For photograph materials or optical materials, a cellulose acetate filmhaving an average degree of acetylation from 58.0 to 62.5% is commonlyused. A cellulose acetate which has an average degree of acetylation ofnot less than 58% is classified into a cellulose triacetate. Thecellulose acetate film is usually produced by the solvent cast method.In the solvent cast method, a film is formed by casting a solution(dope) obtained by dissolving a cellulose acetate in a solvent on asupport, and evaporating the solvent.

Regarding a cellulose acetate film and a production process of the film,many improved means have been conventionally proposed. A method forpreparing a cellulose acetate solution (dope) has been recently proposedwhich comprises cooling a mixture of a cellulose acetate and an organicsolvent, and heating the mixture to dissolve the cellulose acetate inthe organic solvent (Japanese Patent Application Laid-Open No.95544/1997 (JP-9-95544A), Japanese Patent Application Laid-Open No.95557/1997 (JP-9-95557A), and Japanese Patent Application Laid-Open No.95538/1997 (JP-9-95538A), U.S. Pat. No. 5,663,310, and U.S. Pat. No.5,705,632). According to the method comprising the cooling step and thewarming step (hereinafter, the method is referred to as a coolingdissolution method), a solution can be prepared even in the case ofusing a combination an organic solvent and a cellulose acetate insolublein the organic solvent in a conventional manner. The cooling dissolutionmethod is effective means in the case where a film is made from acellulose triacetate (having an average degree of acetylation of notless than 58%) having a low solubility.

As mentioned above, the cooling dissolution method has been developedfor producing a film from a cellulose triacetate (having an averagedegree of acetylation of not less than 58%) having a low solubility.However, in the thickness direction's retardation value (Rt) isincreased in a film produced from a cellulose acetate having an averagedegree of acetylation from 55.0 to 58.0%. Further, the use of thecooling dissolution method ensures a higher retardation value. Moreover,in the casting process, a low residual amount of the solvent and areleasing tension as low as possible in releasing from a metal supportensures a film suitable for a WV film having a high Rt.

The degree of acetylation represents the amount of acetic acid bound andis the weight percentage of bound acetic acid per weight of celluloseunit. The degree of acetylation can be determined in accordance with theprocedure for determination of the degree of esterification as set forthin ASTM D-817-91 (test methods for cellulose acetate etc.). The testprotocol is as follows. Weigh exactly 1.9 gram of a cellulose acetate,previously dried, dissolve in 150 ml of a mixture of acetone-dimethylsulfoxide (4:1, v/v), add 30 ml of an aqueous 1N-sodium hydroxidesolution, and saponify at 25° C. for 2 hours. Add phenolphthalein as anindicator and titrate the excess sodium hydroxide with 1N-sulfuric acid(concentration factor: F). Perform a blank test in the same manner andcalculate the degree of acetylation by means of the following equation.Degree of acetylation (%)=[6.5×(B−A)×F]/W

wherein “A” represents an amount of 1N-sulfuric acid (ml) added to thesample, “B” represents an amount of 1N-sulfuric acid (ml) added toblank, “F” shows a concentration factor of 1N-sulfuric acid, and “W” isa weight of the sample.

Further, the degree of polymerization of the cellulose acetate, in termsof viscosity average degree of polymerization, is about 200 to 400,preferably about 250 to 400, and more preferably about 270 to 400 (e.g.,about 290 to 400). The viscosity average degree of polymerization isusually about 270 to 350. The average degree of polymerization may bedetermined in accordance with the intrinsic viscosity method of Uda etal. (kazuo Uda, and Hideo Saito, Journal of The Society of Fiber Scienceand Technology, Japan (Sen-i Gakkaishi), Vol. 18, No. 1, page 105 to120, 1962). In this method, a solvent may be selected depending on thedegree of acetylation of a cellulose acetate, and others. For example,in the case of a cellulose triacetate, the test protocol is as follows.Dissolve a cellulose triacetate in a mixture of methylenechloride/methanol (9/1, by weight) at a predetermined concentration c(2.00 g/L). Then, inject the solution into an Ostward viscosimeter andmeasure the flow time (in seconds) “t” of the solution between theviscosimeter graduations at 25° C. On the other hand, measure the blankflow time (in seconds) “to” with the above solvent mixture and calculatethe viscosity average degree of polymerization by means of the followingformula.η_(rel) =t/t ₀[η]=(1nη _(rel))/cDP=[η]/(6×10⁻⁴)

wherein “t” shows a flow time (in seconds) of the solution, “t₀” shows aflow time (in seconds) of the solvent, “c” represents a cellulosetriacetate concentration of the solution (g/L), η_(rel) represents arelative viscosity, [η] is an intrinsic viscosity, and DP is an averagedegree of polymerization.

When methylene chloride/methanol=9/1 (by weight) is used as the solventmixture, for instance, the viscosity of a 6% by weight solution ofcellulose triacetate may for example be about 200 to 700 cps (mPa·s),preferably about 250 to 600 cps, and particularly about 250 to 500 cps.

Secondly, the embodiment (2) of the cellulose ester is described.

A mixed fatty acid ester of cellulose means a product obtained byesterifying a cellulose with a mixed fatty acid other than acetic acidin esterification providing a cellulose ester. That is, the mixed fattyacid ester of cellulose may be synthesized by using an acid anhydride oran acid chloride as an acylating agent. In the case where the acylatingagent is an acid anhydride, an organic acid (e.g., acetic acid) ormethylene chloride is used as a reaction solvent. As a catalyst, aprotic catalyst such as sulfuric acid is employed. In the case of usingan acid chloride as the acylating agent, a basic compound is used as acatalyst.

In the industrially most common synthesis method, a cellulose isesterified with a mixed organic acid component containing an organicacid corresponding to an acetyl group and other acyl groups (aceticacid, propionic acid, butyric acid) or an acid anhydride thereof (aceticanhydride, propionic anhydride, butyric anhydride) to synthesize acellulose ester. In this method, a cellulose such as a cotton linter(linter pulp) or a wood pulp is activated with an organic acid such asacetic acid, and then esterified with the use of the mixture of theorganic acid component as described above in the presence of a sulfuricacid catalyst.

The organic acid anhydride component is generally used in an excessiveamount relative to the amount of the hydroxyl group present in thecellulose. In the esterification treatment, a hydrolysis reaction(depolymerization) of the cellulose main chain (β1→4 glycoside linkage)proceeds in addition to the esterification reaction. Proceeding of thehydrolysis reaction in the main chain contributes to reduction of thepolymerization degree of the cellulose ester, and the produced celluloseester film is deteriorated in the physical properties. It is necessaryto decide the reaction conditions such as a reaction temperature inconsideration of a polymerization degree or a molecular weight of theresulting cellulose ester. Needless to say, a cellulose ester can beproduced with the use of only organic acid(s) other than acetic acid asan organic acid. However, a film made of the obtained cellulose ester,for example, a cellulose propionate or a cellulose butyrate, is oflittle practical use because such a film is inferior to the celluloseacetate film in mechanical strength or durability. The most appropriatealternative for a cellulose acetate includes a mixed fatty acid ester ofcellulose esterified with organic acids corresponding to acyl groupsincluding acetyl group. These mixed fatty acid esters of cellulose aredescribed in Japanese Patent Application Laid-Open No. 45804/1998(JP-10-45804A).

The specific examples of the mixed fatty acid ester of cellulose includea cellulose acetate C₃₋₆acylate such as a cellulose acetate propionateand a cellulose acetate butyrate, preferably a cellulose acetateC₃₋₄acylate, and others.

Thirdly, the embodiment (3) of the cellulose ester is described.

As mentioned above, in the case of using the cellulose ester as a liquidcrystal protective film, Rt is required to be as low as possible.However, there is a problem that a cellulose triacetate solutionobtained by the cooling dissolution method is low in stability.Moreover, there is also a problem that a cellulose acetate film producedby the cooling dissolution method has a high Rt value.

In an ordinary synthetic method, a cellulose acetate has higher degreeof substitution at 2- or 3-position than that at 6-position. Therefore,it is necessary that the above-mentioned reaction conditions arespecially adjusted in order to provide the degrees of substitution at2-, 3- and 6-positions of not less than 2.67 maintaining the degrees ofsubstitution at 2- and 3-positions of not more than 1.95.

As the specific reactive condition, it is preferred to reduce the amountof the sulfuric acid as a catalyst, and to extend the time for theacetylation (or acetification) reaction. The large amount of thesulfuric acid catalyst hastens progress of the acetylation reaction,however, depending on the amount of the catalyst, the cellulose ester isesterified with the sulfuric acid catalyst. On the completion of thereaction, followed by removing the sulfuric acid group from the ester, aresidual hydroxyl group occurs. A larger amount of sulfuric acid esteris formed at 6-position having a higher reactivity. Accordingly, whenthe amount of the sulfuric acid catalyst is large, the degree ofsubstitution at 6-position is small. Therefore, in order to synthesize acellulose acetate used in the present invention, there can be cited aproduction method of reducing the amount of the sulfuric acid catalystas much as possible, and extending the reaction time to cover thelowered reaction rate due to reduction of the catalyst. At the sametime, specifically, it is preferred to produce the cellulose acetate inaccordance with a method described in Japanese Patent ApplicationLaid-Open No. 338601/2002 (JP-2002-338601A). That is, the followingproduction method is available: a method which comprises a step forallowing a cellulose to react with acetic acid or acetic anhydride in asolvent in the presence of a catalyst (sulfuric acid) to synthesize acellulose acetate, and a step for ripening the cellulose acetate in thepresence of an acetyl group donor, water or an alcohol (0.1 to 10 mol %)relative to the acetyl group donor and a catalyst (sulfuric acid).

The inventors of the present invention found that such a celluloseacetate having a high degree of substitution at 6-position and having aratio of sulfuric acid group (sulfo group) relative to the amount ofcalcium in a specific range is particularly suitably used in the presentinvention for dissolving in a solvent other than methylene chloride, andthat such a cellulose acetate can easily obtain a cellulose acetatesolution which strikes a balance between heat resistance under wetcondition and releasability from a metal support in a solution casting.

The degrees of substitution at 2-, 3- and 6-positions in the celluloseacetate may be determined by propionylating the cellulose acetate andmeasuring the resulting product by ¹³C-NMR. The measuring method isdetailed in Tezuka, et al (Carbohydr. Res. 273 (1995) 83-91). Thesecellulose acetates increased in the degree of substitution at 6-positionis detailed in Japanese Patent Application Laid-Open No. 5851/1999(JP-11-5851A) and Japanese Patent Application Laid-Open No. 338601/2002(JP-2002-338601A).

A cellulose acetate in which the degrees of substitution at 2- and3-positions are not less than 1.70 and not more than 1.95 and the degreeof substitution at 6-position is not less than 0.88 is preferred. Morepreferred cellulose acetate is one in which the degrees of substitutionat 2- and 3-positions are not less than 1.80 and not more than 1.95 andthe degree of substitution at 6-position is not less than 0.88 and notmore than 0.95. Further preferred cellulose acetate is one in which thedegrees of substitution at 2- and 3-positions are not less than 1.84 andnot more than 1.92 and the degree of substitution at 6-position is notless than 0.89 and not more than 0.92.

Incidentally, in the present invention, 1 gram of a cellulose estermeans 1 gram of the cellulose ester in absolute dry condition. Since acellulose ester being usually in a flaky or granulated form contains 20to 40% of water, it is necessary to remove the water from the celluloseester before measurement. Moreover, since an optical film or the likeusing a cellulose ester contains a plasticizer and a additive, thecellulose ester can be obtained by removing these plasticizer andadditive with the use of a separation method which comprises immersingthe film in an organic solvent with utilizing the difference in thesolubility. The manner for removing these plasticizer and additive isdetailed in Polymer Analysis Handbook edited by The Japan Society forAnalytical Chemistry (issued by Asakura Shoten Co., Ltd.).

Incidentally, in the above embodiment (3) concerning a cellulosetriacetate having a specific distribution in the degree of substitution,as far as the cellulose triacetate has the specific distribution in thedegree of substitution, the production process or the degree ofacetylation may be the same as that of the above embodiment (1).

In the next place, the stabilization treatment is mentioned.

As described above, in the general production method of a celluloseester, a raw material is allowed to react with an acid such as aceticanhydride by using acetic acid as a solvent, and at that time, a strongacid having a dehydrating action is utilized as a catalyst. As thecatalyst, the sulfuric acid is usually employed, and sulfuric acid notonly acts as a catalyst but also forms a cellulose sulfate. As a result,in some reactions, the sulfuric acid group remains also in the reactionproduct. Therefore, such a cellulose ester usually contains a remarkablyexcessive amount of a stabilizer, for example, an alkali metal (e.g.,lithium, potassium, and sodium), a salt thereof or a compound thereof,or an alkaline earth metal (e.g., calcium, magnesium, strontium, andbarium), a salt thereof or a compound thereof, in order to improve inheat resistance or heat resistance under wet condition. Therebystability is imparted to the cellulose ester without the release ofsulfuric acid group (sulfo group).

Thus it is arbitrary that the cellulose ester contains an alkali metalor an alkaline earth metal, and the objective to put such a metal is toimprove heat stability or heat resistance under wet condition asdescribed above. However, at the same time, it is also confirmed thatthese alkali metal and alkaline earth metal have an influence onreleasability from a metal support in a casting process and spinnability(Japanese Patent Application Laid-Open No. 316701/1998 (JP-10-316701A)).In the case where the amount of the alkaline earth metal to be added isreduced, the cellulose ester is inferior in stability, and heatresistance under wet condition thereof is unsatisfactory. That is, it isrecognized that heat resistance under wet condition is not compatible toreleasability from a metal support in a casting process (orspinnability).

However, the inventors of the present invention found that each elementin the alkaline earth metals has a different effect corresponding to theamount to be added from others, that among the alkali metals and thealkaline earth metals, calcium specifically has an effect on heatresistance under wet condition, and that the ratio of calcium relativeto the remaining sulfuric acid (or sulfuric acid group) is related toheat resistance under wet condition or releasability from a metallicmaterial.

That is, the embodiments of the present invention are described asfollows:

(a) a cellulose ester in which the molar ratio determined from (A) thetotal amount (in terms of molar amount) of residual sulfuric acid in 1gram of the cellulose ester and (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula:0.5<(B)/(A)<1.5

[e.g., 0.6<(B)/(A)<1.4],

(b) a cellulose ester in which the molar ratio determined from (A) thetotal amount (in terms of molar amount) of residual sulfuric acid in 1gram of the cellulose ester and (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula:0.5<(B)/(A)<1.2

[e.g., 0.65<(B)/(A)<1.2],

(c) a cellulose ester in which the molar ratio determined from (A) thetotal amount (in terms of molar amount) of residual sulfuric acid in 1gram of the cellulose ester and (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula:0.6<(B)/(A)<1.0

[e.g., 0.7<(B)/(A)<1.0], and

(d) a cellulose ester in which the molar ratio determined from (A) thetotal amount (in terms of molar amount) of residual sulfuric acid in 1gram of the cellulose ester and (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula:0.75<(B)/(A)<1.0

[e.g., 0.8<(B)/(A)<1.0].

Incidentally, in the cellulose ester (particularly, a celluloseacetate), the amount of the residual sulfuric acid may be adjusted bythe amount of sulfuric acid to be used, and others. The total amount ofresidual sulfuric acid in 1 gram of the cellulose ester may be, forexample, about 0.1×10⁻⁷ to 1000×10⁻⁷ mol, preferably about 1×10⁻⁷ to500×10⁻⁷ mol (e.g., about 5×10⁻⁷ to 300×10⁻⁷ mol), and more preferably10×10⁻⁷ to 100×10⁻⁷ mol (e.g., about 10×10⁻⁷ to 50×10⁻⁷ mol).

Moreover, the present invention may be the above-mentioned celluloseester (particularly a cellulose acetate) (a), (b), (c) or (d); and thecellulose ester is the following cellulose ester (e) or (f) whichcontains a specific amount of calcium shown below;

(e) a cellulose ester in which (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula:5×10⁻⁷<(B)<20×10⁻⁷, or

(f) a cellulose ester in which (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of the cellulose ester fulfillsthe following formula;8×10⁻⁷<(B)<15×10⁻⁷.

The process for adding these stabilizers varies depending on theproduction method of the cellulose ester. That is, the stabilizer isadded as a neutralizing agent in a step for completing theesterification in some cases, or is added in the form of a metal acetatebefore or during ripening step carried out after the esterification inother cases. Moreover, a metal acetate is added in ageing step, and thesame metal acetate is sometimes added after completion of the ripeningstep. Further, the stabilizer is added in the form of an alkali metalion and/or an alkaline earth metal ion in a coagulant (or a coagulator)for precipitation of a cellulose ester dope after ripening in somecases. Furthermore, in washing of a cellulose ester obtained bycoagulation and precipitation, the above-mentioned ion is added insometimes. Additionally, in some cases, a solution of a water-solublealkali metal or alkaline earth metal salt is added to a dried celluloseester flake or particle by spraying or other means. These methods aresometimes used in combination.

According to the present invention, in the above-mentioned productionprocess of a cellulose ester (cellulose acetate, particularly cellulosetriacetate), which comprises acylating (particularly acetylating with anacetylating agent) a cellulose in the presence of sulfuric acid, andhydrolyzing (or deacylating) [particularly ripening] the resultingmatter, sulfuric acid as a catalyst usually is neutralized at least inpart with a neutralizing agent [in particular a magnesium component suchas magnesium oxide, magnesium hydroxide, and a magnesium salt (e.g., amagnesium salt of an inorganic acid such as magnesium carbonate, and amagnesium salt of an organic acid such as magnesium acetate)], and thenin an appropriate step, a calcium component as a heat stabilizer[particularly, for example, calcium oxide, calcium hydroxide, a calciumsalt (e.g., a calcium salt of an inorganic acid such as calciumcarbonate, and a calcium salt of an organic acid such as calciumacetate)] in many cases.

The heat stabilizer (calcium component) may be usually added afteracylating (or after esterifying, particularly after acetizing) acellulose. For example, the heat stabilizer may be added (I) beforehydrolysis (ripening) or (II) in a course after ripening before a stepof precipitation, or in both timing.

The most preferred method of the present invention includes a methodwhich comprises

acylating (particularly, acetylating) a cellulose with an acetylatingagent in the presence of sulfuric acid,

neutralizing the sulfuric acid at least in part with a neutralizingagent [e.g., a neutralizing agent containing at least a magnesiumcomponent (e.g., magnesium acetate)] in an appropriate step after theacylating (in particular, acetylating or acetizing) step [for example,(i) after completing acylating or esterifying (acetizing) step (that is,in a course after acylating (particular acetylating) step beforehydrolysis (or ripening)), and/or (ii) in a course after ripening stepbefore adding the calcium component], and

adding the calcium component [for example, calcium hydroxide and/or acalcium salt, particularly calcium hydroxide] in the subsequent step ofthe neutralizing step [for example, after the ripening step (e.g., thecoagulating and precipitating step, and the washing)]. By such a method,the ratio of the total amount of sulfuric acid relative to calciumremaining in the cellulose ester can be efficiently adjusted to theabove-mentioned range (e.g., 0.5<(B)/(A)<1.5). The concentration oramount of these neutralizing agent and stabilizer may be determined inconsideration of a calcium content in a final cellulose ester.

The magnesium content in the cellulose ester (particularly the celluloseacetate) depends on the kind or species of the raw material, or thepossibility of magnesium component usage. Even in the case of using awood pulp (e.g., a hardwood pulp), the magnesium content in 1 gram ofthe cellulose ester (converted into weight) may be usually selected fromnot more than 30 ppm (e.g., about 0 to 30 ppm), and may for example be 0to 25 ppm, preferably not more than 20 ppm (e.g., about 0.1 to 20 ppm),and more preferably not more than about 15 ppm (e.g., about 0.3 to 12ppm), or may be not more than about 10 ppm (e.g., about 0.5 to 9 ppm),in 1 gram of the cellulose ester.

Examples of the method for reducing the amount of sulfuric acid in thecellulose ester include the following method;

(1) adding water in the end of acetylation reaction,

(2) slowly adding water in the neutralization,

(3) decreasing the amount of sulfuric acid as a catalyst, and

(4) increasing a temperature before the neutralization or during theripening.

If necessary, these methods may be suitably used in combination toreduce the amount of the sulfuric acid.

For achieving the objects of the present invention as mentioned above,the ratio of calcium relative to the sulfuric acid group becomes anissue. The molar number of calcium is needed to be at least 0.5 timerelative to the molar number of the sulfuric acid group. In theconventional manner, an alkali metal and/or an alkaline earth metal wasadded in a remarkably excessive amount as compared with the amount ofthe sulfuric acid group. This is due to a partial carboxylation of ahydroxyl group in the cellulose in the process of esterification. Sincesuch a carboxyl group is present, an alkali metal and/or an alkalineearth metal is added in about double amount of the sulfuric acid group.According to the experiments of the inventors of the present invention,in the case where the molar number of calcium is over 0.5 time relativeto that of the sulfuric acid group, required heat resistance under wetcondition can be obtained. More preferably, the molar number of calciumis over 0.6 time relative to that of the sulfuric acid group.Furthermore, for releasability from a metal support in the casting, itis necessary that the molar number of calcium should be less than 1.5times relative to the molar number of the sulfuric acid group. Morepreferably, the molar number of calcium should be less than 1.0 timerelative to that of the sulfuric acid group.

As described above, as far as the sulfuric acid catalyst is used in theesterification step in the cellulose ester, the residual sulfuric acidgroup cannot be perfectly removed even when the cellulose ester isproduced by using the most suitable production process for the aspect ofthe cellulose ester in the present invention. The residual sulfuric acidgroup is at the lowest about 10×10⁻⁷ mol per gram of the celluloseester. Therefore, it is preferred to contain about 5×10⁻⁷ mol of calciumper gram of the cellulose ester. Moreover, in the case where the calciumcontent is over about 20×10 ⁷ mol, a problem may occur with thereleasability from a metal support in the casting process andspinnability (or spinning property). As the preferred range, the calciumcontent (mol) is not less than 5×10 and not more than 20×10 per gram ofthe cellulose ester. As a particularly preferred range, the calciumcontent (mol) is not less than 8×10 and not more than 15×10 per gram ofthe cellulose ester.

It is not clear why calcium and the residual sulfuric acid group do nothave an effect on the releasability from a metal support in the castingprocess and spinnability when the amount of calcium has a specific ratiorelative to the residual sulfuric acid group. The reason is supposed asfollows. That is, a carboxyl group or a sulfonic acid group (sulfogroup) is bonded to the cellulose ester in the esterification step, andcalcium ion or magnesium ion is neutralized in the neutralization step,wherein the calcium ion is selectively bonded to the sulfonic acidgroup. Thereby, false adhesion of the cellulose ester to a metallicmaterial is inhibited by interposing the calcium atom, and as a resultthe calcium does not exert an influence on the releasability and thespinnability.

As an analysis method of a calcium content in such a cellulose ester, aknown method may be utilized. That is, the calcium content may bequantitatively determined by conducting a pretreatment called as theashing method which comprises burning a cellulose ester completely anddissolving the resulting ash in hydrochloric acid, and analyzing thepretreated matter through an atomic absorption spectrometry. Moreover,the calcium content may be determined by pre-treating an absolutelydried cellulose ester with Advanced Microwave Labstation (sulfonitricacid decomposition) and an alkali melting, and analyzing the pretreatedmatter with ICP-AES (inductively coupled plasma atomic emissionspectrometer).

Such a cellulose ester is high in releasability (or is easy to release)from a base support in production of a film by the casting process.Moreover the cellulose ester is excellent in optical characteristicssuch as transparency. The transparency of the cellulose ester is, forexample, about 60 to 100% (preferably about 70 to 100%, more preferablyabout 75 to 100%), and usually about 70 to 90%. The haze of thecellulose ester is about 1 to 8 (preferably about 1 to 5). Further, ayellowness index (YI) which is an index of yellowness of a celluloseester is, for example, about 1 to 10. Incidentally, the transparency,the haze and the yellowness index (YI) may be measured according to thefollowing manner.

[Transparency]

Weigh exactly 8.0 gram of a cellulose ester, previously dried, add 125.3gram of a solvent (e.g., a mixture of methylene chloride-methanol (9:1,w/w) or acetone), and effect thorough dissolution (sample solutionhaving a concentration of 6% by weight). Using an AKA photoelectriccalorimeter equipped with a cesium photoelectric tube and a filter No.12, put the solvent in the glass cell with a light path length of 100 mmand measure the blank transmittance. Then, put the 6% by weight samplesolution in the glass cell with a light path length of 100 mm andmeasure its transmittance. With the blank transmittance value beingtaken as 100%, the percentage of the transmittance value of the samplesolution is recorded as the transparency of the sample.

[Haze]

Weigh exactly 12.0 gram of a cellulose ester, previously dried, add 88.0gram of a solvent (e.g., a mixture of methylene chloride-methanol (9:1,w/w) or acetone), and effect thorough dissolution (sample solutionhaving a concentration of 12% by weight). Using a turbidimeter (NipponDenshoku Kogyo Co., Ltd.) and a glass cell (45 mm wide, 45 mm high,light path length 10 mm), perform measurements as follows. Put thesolvent in the glass cell, set the cell in the turbidimeter, and performzeroing and calibration. Then, put the 12% by weight sample solution inthe glass cell, set the cell in the turbidimeter, and record thereading.

[Yellowness Index (YI)]

Weigh exactly 12.0 gram of a cellulose ester, previously dried, add 88.0gram of a solvent (e.g., a mixture of methylene chloride-methanol (9:1,w/w) or acetone), and effect thorough dissolution (a solution in aconcentration of 12% by weight). Using a color difference meter (NipponDenshoku Kogyo Co., Ltd., Color Difference Meter Σ90) and a glass cell(45 mm wide, 45 mm high; light path length 10 mm), perform measurementsand calculate YI by means of the following equation.YI=YI2−YI1

wherein YI1 represents YI for the solvent, and YI2 shows YI for the 12%by weight sample solution.

Further, the above cellulose esters have a high solution stability,filterability and spinnability, and thus remarkably reduce the frequencyof thread breakage in a continuous spinning for a long time.

Moreover, the cellulose ester of the present invention is high in heatresistance under wet condition as described above. The present inventiontherefore also includes, as described above, a method for improving heatresistance under wet condition of a cellulose ester, which comprisesadjusting (A) the total amount (in terms of molar amount) of residualsulfuric acid in 1 gram of the cellulose ester and (B) the total amount(in terms of molar amount) of calcium contained in 1 gram of thecellulose ester so that the molar ratio (B)/(A) is in the range of theabove-mentioned formulae (e.g. 0.5<(B)/(A)<1.5).

Incidentally, the heat resistance under wet condition may be determinedas follows.

About 2.0 gram of a cellulose ester, previously dried and pulverized, isweighed in a heat-resistant glass tube, and 2 ml of a distilled water isadded thereto. The glass tube is sealed, and put in a pure water(boiling water) for 7 hours. After cooling, the content is washed outwith a boiling (pure) water on a filter paper, and the filtrate is madeinto 150 ml in total. This filtrate is titrated with 0.01N-NaOH solution(aqueous solution) by using phenolphthalein as an indicator. At the sametime, a blank test is carried out with the use of only pure water, andheat resistance under wet condition is calculated based on the followingformula:Heat resistance under wet condition (%)=(A−B)×F×0.06/sample weight (gr.)

wherein “A”: a titer (ml) of 0.01N-NaOH solution (aqueous solution),“B”: a titer (ml) of 0.01N-NaOH solution (aqueous solution) in the blanktest, and “F”: a factor of 0.01N-NaOH solution (aqueous solution).

The heat resistance under wet condition (%) of the above-mentionedcellulose ester measured through the above method is not more than about0.08% (e.g., about 0.01 to 0.08%), preferably not more than about 0.07%(e.g., about 0.02 to 0.065%), more preferably not more than about 0.06(e.g., about 0.02 to 0.055%), and particularly about not more than 0.05%(e.g., about 0.02 to 0.05%).

The cellulose acetate of the present invention having such properties isuseful for preparing a cellulose acetate solution (dope) and producing afilm or a fiber.

(i) The dope of the present invention comprises at least a celluloseacetate among the above-mentioned cellulose esters in embodiments (1) to(3). (ii) The dope of the present invention belongs to at least one ofthe above-mentioned embodiments (1) to (3) of the present invention.Moreover, a mixture of the embodiments (1) to (3) of the celluloseesters of the present invention obtained by mixing a plurality ofcellulose esters can also belong to at least one of the above-mentionedembodiments (1) to (3) of the present invention.

The dope usually comprises a cellulose ester and a solvent (an organicsolvent). Depending on the average degree of acetylation in thecellulose ester and others, the solvent may for example be selected froma halogenated hydrocarbon (e.g., methylene chloride, and ethylenechloride), a ketone (e.g., acetone, methyl ethyl ketone, methyl isobutylketone, and cyclohexanone), an ester (e.g., a formic ester such as ethylformate, an acetic ester such as methyl acetate or ethyl acetate, andethyl lactate), an ether (e.g., dioxane, and dimethoxyethane), acellosolve (e.g., methyl cellosolve, and ethyl cellosolve), a cellosolveacetate (e.g., methyl cellosolve acetate, and ethyl cellosolve acetate),and a mixture of these compositions. The solvent may comprise a nitrocompound (e.g., nitromethane, nitroethane, and nitropropane), a loweralcohol (e.g., methanol, ethanol, isopropanol, butanol, and diacetonealcohol), and others.

In the case of using a cellulose ester of the embodiment (2) of thepresent invention, a stable dope can be obtained without a halogenatedhydrocarbon as a solvent. Moreover, in the case of using a celluloseester of the embodiment (3) of the present invention, the celluloseester can be dissolved in an organic solvent by cooling the solvent evenif a halogenated hydrocarbon is not used as a solvent.

The amount to be used of the solvent may be selected from such a rangein which casting property in film forming, spinnability, handleability,and others are not deteriorated. For example, the amount of the solventis about 150 to 1000 parts by weight (concentration of celluloseester=about 10 to 40% by weight), and preferably about 200 to 900 partsby weight (concentration of cellulose ester=about 10 to 30% by weight),relative to 100 parts by weight of the cellulose ester. The content ofthe cellulose ester is usually about 10 to 25% by weight (e.g., about 10to 20% by weight). Thus obtained dope is useful for forming a film bythe casting process.

In the film casting, usually a cellulose diacetate or a cellulosetriacetate (particularly a cellulose triacetate) is employed. A filmthrough the dope casting is usually obtained by flow-casting a dope to asupport and partially drying the dope, releasing it from the support,and drying it. As the support (or base material), there may be used aconventional support, for example, a mirror-finished metal support(e.g., a stainless-steel support), and others.

As described above, since the cellulose ester of the present inventionis high in releasability (releasing property) from the support, asemi-dried film can be smoothly released from the support, and acellulose ester film having a high surface smoothness can be obtained.Therefore, the method using the dope of the present invention is usefulas a method improving the releasability of film from support.

Moreover, the cellulose ester film of the present invention is excellentin optical characteristics (e.g., yellowness index (YI), haze, andtransparency) because the film comprises the above-mentioned celluloseester. The thickness of thus obtained cellulose ester film may beselected depending on its usage from a range of e.g., about 5 to 500 μm,preferably about 10 to 200 μm, more preferably about 20 to 140 μm, andfurther preferably about 30 to 130 μm (particularly about 50 to 120 μm).Incidentally, the cellulose ester of the present invention is not onlyuseful for forming an optical film (e.g., a photographic film, aprotective film for a polarizer, a retarder film, a scattering film, awide-view-angle film (a WV film), and a film for a color filter) by thecasting process, but also utilized for producing a thin optical film bya spin-coating method because the cellulose ester is excellent inreleasability from the support.

The cellulose ester of the present invention is also superior instability, filterability, and spinnability of the dope as describeabove. When a fiber is produced by spinning process, a cellulosediacetate or a cellulose triacetate is usually employed as the celluloseester. The spinning process may be carried out by a conventional method,for example, by spinning a dope from a spinneret having many fine holesand drying the spinning yarns or threads, and if necessary drawing orstretching the fibers. The use of the dope of the present invention inthe spinning process may prevent clogging or choking of the holes orthreads for a long time. Therefore, the method with use of the dope ofthe present invention is useful for improving spinnability.

The cellulose ester or the dope of the present invention may contain aplasticizer, for example, a phosphoric ester such as triphenyl phosphate(TPP) and tricresyl phosphate (TCP), a phthalic ester such as dimethylphthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP),dioctyl phthalate (DOP) and di-2-ethylhexyl phthalate (DEHP), a fattyacid ester such as butyl oleate, methylacethyl ricinoleate and dibutylsebacate, a citric ester such as acetyltriethyl citrate (OACTE) andacetyltributyl citrate (OACTB), a trimellitic ester, and others. Theseplasticizers may be used singly or in combination. The cellulose estermay contain an aging inhibitor, for example, an antioxidant, anultraviolet ray absorbing agent, a peroxide decomposer, a radicalscavenger, a metal deactivator or sequestrant, an acid acceptor, andothers. Further, if necessary, the cellulose ester may contain otheradditive(s), for example, a nucleating agent, an inorganic powder (e.g.,a diatomaceous earth (or a diatomite), calcium carbonate, and titaniumoxide), a thermal stabilizer (or a heat stabilizer), a flame retardant,a coloring agent, and others.

INDUSTRIAL APPLICABILITY

The use of the cellulose ester of the present invention (a celluloseester in which the contents of the alkali metal and the alkaline earthmetal are in a specific range) ensures obtaining a film which has a highreleasability from a support in the casting process and is excellent insurface smoothness and optical characteristics with being excellent inheat resistance under wet condition. Moreover, the cellulose ester ofthe present invention is also excellent in stability of the dope,filtrating property or spinnability and heat stability.

EXAMPLES

The following examples are intended to describe the present invention infurther detail and should by no means be interpreted as defining thescope of the invention. Incidentally, the reaction temperature and time,catalyst amount and others described in the Examples are limited to thecase conducted by the apparatus of the inventor of the presentinvention. The reaction of the cellulose ester is a complicatedsolid-liquid reaction, and the results naturally vary depending on thesize, shape and temperature-measuring site of the reactor, and otherfactors. Since important factors are a Ca/SO₄ ratio disclosed in thepresent invention and an amount of calcium, it goes without saying thatother conditions should be set to optimum conditions to ensure necessaryphysical properties in response to the reactor.

[Releasability]

Incidentally, the releasability of the film from the support in thecasting process was evaluated as follows. A cellulose ester (100 partsby weight) was mixed with 320 parts by weight of methylene chloride, 40parts by weight of methanol, 25 parts by weight of butanol and 15 partsby weight of triphenyl phosphate (TPP) to prepare a dope. The dope wascast on a smooth stainless-steel plate (support) in a thickness of about1 mm at a room temperature (20 to 25° C.), and allowed to stand at theroom temperature for 3 to 4 minutes. Then, the releasability of the filmfrom the support was evaluated based on the following criteria.

“A”: Smoothly released with a minimum of peel drag and the film surfacewas smooth.

“B”: Not smoothly released but a considerable peel drag was felt, filmpeelings remain on the stainless steel plate.

[Content of Alkaline Earth Metal]

A dried cellulose ester was completely burned, and then the resultingash was dissolved in hydrochloric acid. After such a pretreatment, thecontent of the alkaline earth metal was measured by the atomicabsorption spectrometry. The measured value is obtained as the contentof each alkaline earth metals in 1 gram of the cellulose ester in anabsolute dry condition at ppm unit.

[Amount of Sulfuric Acid]

The total sulfuric acid in the cellulose ester was determined bycombusting a dried cellulose ester in an electric furnace at 1300° C.,trapping the evolved sulfurous acid gas in 10% hydrogen peroxide-water,and titrating it with a normalized sodium hydroxide solution (a sodiumhydroxide solution having a given normality, usually 0.01 to 1N sodiumhydroxide solution). The data are in terms of SO₄ ²⁻. The content ofsulfuric acid is obtained as the determined value in 1 gram of thecellulose ester in an absolute dry condition at ppm unit.

[Ca/SO₄ Ratio]

A Ca/SO₄ ratio is calculated from the calcium content among the contentsof the alkaline earth metals and the amount of sulfuric acid. The Ca/SO₄ratio is molar ratio. Namely, by dividing the amount of sulfuric acid by96, (i) the sulfuric acid content in 1 gram of the cellulose ester isobtained in mol unit. In the same way, (ii) by dividing the calciumcontent among the contents of the alkaline earth metals by 40.1, thecalcium content in 1 gram of the cellulose ester is obtained in molunit. Accordingly, by dividing (ii) by (i), the Ca/SO₄ ratio can bedetermined.

[Heat Resistance Under Wet Condition]

A cellulose ester (about 2.0 g), previously dried and pulverized, wasweighed in a heat-resistant glass tube, and 2 ml of a distilled waterwas added to the glass tube. Then the glass tube was sealed and put in apure water (boiling water) for 7 hours. After cooling, the content waswashed out with boiling water on a filter paper, and the filtrate wasmade into 150 ml in total. This filtrate was titrated with 0.01N-NaOHsolution (aqueous solution) by using phenolphthalein as an indicator. Atthe same time, a blank test is carried out with the use of only purewater, and heat resistance under wet condition was calculated based onthe following formula:Heat resistance under wet condition (%)=(A−B)×F×0.06/sample weight (gr.,or gram (g))

wherein “A”: a titer (ml) of 0.01N-NaOH solution, “B”: a titer (ml) of0.01N-NaOH solution (aqueous solution) in the blank test, and “F”: afactor of 0.01N-NaOH solution (aqueous solution).

The heat resistance under wet condition is an index indicatingdifficulty of hydrolysis of a cellulose ester in the case where thecellulose ester has enough moisture and is exposed to a hightemperature. When the index of a cellulose ester is not more than 0.08%,the cellulose ester is evaluated as having stability. That is, in thecase where a film is made from the obtained cellulose acetate with theabove-mentioned method, problems accompanied with hydrolysis of thecellulose ester hardly occur even when the cellulose ester is maintainedfor a long period (e.g., 100 hours) under a condition of hightemperature and high humidity (e.g., 40° C. and 90RH %).

Examples 1, 2 and 5 and Comparative Examples 1 to 3

[Preparation of Cellulose Acetate]

One hundred (100) parts by weight of a hardwood kraft pulp (α-cellulosecontent 98.5%) was sprayed with 50 parts by weight of glacial aceticacid for activation. Then, a mixture of 445 parts by weight of glacialacetic acid, 265 parts by weight of acetic anhydride and 8.3 parts byweight of sulfuric acid was added to the activated mixture, and theesterification was carried out in the conventional manner. Thereafter,the hydrolysis reaction was carried out, magnesium acetate was added asa neutralizing agent to the reaction mixture, and the ripening wasconducted for about 70 minutes at 60 to 70° C. Further, the resultantwas discharged in diluted acetic acid to precipitate a raw cellulosetriacetate. Thus obtained precipitate was divided into 6 parts, and thesolid and the liquid components in each parts were separated from eachother by dehydration and washing with a purified water to give apurified cellulose triacetate (CTA) having a degree of acetylation(average degree of acetylation) of 60.8% and a viscosity-average degreeof polymerization of 313, and each CTA obtained from the above 6 partswas used in Examples 1, 2 and 5 and Comparative Examples 1 to 3,respectively.

[Post-Treatment]

Each flake of the above-mentioned cellulose triacetate was dipped fortreatment with calcium hydroxide aqueous solutions each having differentconcentrations of 5 to 30 ppm, respectively. Then, each of the treatedflakes was filtrated off and dried to give a cellulose triacetatecontaining metal components described in Table 1. Further, Table 1 showsevaluation results of heat resistance under wet condition and thereleasability of a film by the casting process.

Examples 6 to 9

Examples 6 to 9 are described below.

[Preparation of Cellulose Acetate]

In the same manner as in Example 1, one hundred (100) parts by weight ofa hardwood kraft pulp (α-cellulose content 98.5%) was sprayed with 50parts by weight of glacial acetic acid for activation. Then, a mixtureof 445 parts by weight of glacial acetic acid, 265 parts by weight ofacetic anhydride and 8.3 parts by weight of sulfuric acid was added tothe activated mixture, and the esterification was carried out in theconventional manner. Thereafter, the hydrolysis reaction was carriedout, magnesium acetate was added as a neutralizing agent to the reactionmixture, and the ripening was conducted for about 70 minutes at atemperature about 10° C. higher than that of Example 1. Further, theresultant was discharged in diluted acetic acid to precipitate a rawcellulose triacetate. Thus obtained precipitate was divided into 4parts, and the solid and the liquid components in each parts wereseparated from each other by dehydration and washing with a purifiedwater to give a purified cellulose triacetate (CTA) having a degree ofacetylation (average degree of acetylation) of 60.8% and aviscosity-average degree of polymerization of 303, and each CTA obtainedfrom the above 4 parts was used in Examples 6 to 9, respectively.

[Post-Treatment]

Each flake of the above-mentioned cellulose triacetate was dipped fortreatment with calcium hydroxide aqueous solutions each having differentconcentrations of 5 to 30 ppm, respectively. Then, each of the treatedflakes was filtrated off and dried to give a cellulose triacetatecontaining metal components described in Table 1. Further, Table 1 showsevaluation results of heat resistance under wet condition and thereleasability of a film by the casting process. TABLE 1 Heat Amount ofresistance sulfuric under wet Test Ca Mg acid Ca condition Ca MgExamples (ppm) (ppm) (ppm) SO₄ (%) (10⁻⁷ mol) (10⁻⁷ mol) ReleasabilityCom. Ex. 1 1.3 17.0 190 0.02 0.213 0.3 7.0 A Com. Ex. 2 23.3 15.0 1900.29 0.150 5.8 6.2 A Com. Ex. 3 34.7 14.0 200 0.42 0.096 8.7 5.8 A Ex. 140.5 14.0 190 0.51 0.077 10.1 5.8 A Ex. 2 46.7 14.0 200 0.56 0.052 11.75.8 A Ex. 5 66.2 20.0 180 0.88 0.040 16.5 8.2 A Ex. 6 41.5 9.0 100 0.990.046 10.4 3.7 A Ex. 7 31.5 8.7 100 0.75 0.048 7.9 3.6 A Ex. 8 21.4 9.4100 0.51 0.060 5.3 3.9 A Ex. 9 57.5 9.0 100 1.38 0.048 14.3 3.7 A

As apparent from Comparative Examples 1 to 3 and Examples 1 to 2 and 5to 9 in Table 1, the heat resistance under wet condition has aninflection point between the Ca/SO₄ ratio of 0.42 and 0.56. In the casewhere the ratio is not less than 0.50, the value of the heat resistanceunder wet condition may be considered as not more than 0.08%. Asapparent from comparison of Examples 6 to 8 and Comparative Example 3,which have almost the same content of the alkaline earth metals, it isimpossible to ensure an excellent heat resistance under wet conditioneven when the cellulose ester contains an alkaline earth metals of about50 ppm or 14×10⁻⁷ mol per gram of the cellulose ester in dry weight, andit is clear that the ratio between the amounts of sulfuric acid andcalcium is an important factor for heat resistance under wet condition.

Examples 3 and 4

Examples 3 and 4 are described below.

[Preparation of Cellulose Acetate]

(Synthesizing Step)

A wood pulp (water content: 7.3% by weight) containing an α-cellulose inthe amount of about 98.5% was broken into pieces. To 100 parts by weightof the pulp, 30 parts by weight of glacial acetic acid was evenlysprinkled. The resulting mixture was then stirred. After left at a roomtemperature for 90 minutes, the mixture was poured into another mixtureof 270 parts by weight of cooled acetic anhydride, 380 parts by weightof acetic acid and 7 parts by weight of 98% sulfuric acid. Thetemperature of the obtained mixture was linearly elevated for 60 minutesfrom 0° C. (when the reaction was started) to 37° C. by externalintercooling and heating system. The temperature was then kept at 37° C.for 90 minutes, to synthesize a cellulose acetate.

(Ripening Step)

To a dope of the above-prepared cellulose acetate, an aqueous solutionof acetic acid was added. The mixture was heated to 54° C., and kept atthe temperature for 115 minutes to ripen the cellulose acetate. Theamounts of acetic acid (acetyl donor), water and sulfuric acid(catalyst) in the mixture were 1930 parts by weight, 64 parts by weightand 21 parts by weight, respectively, based on 499 parts by weight ofthe cellulose acetate. Accordingly, the ratio of water relative toacetic acid (acetyl donor) was 11 mol %. Thus obtained solution wasmaintained at 30° C. for three hours to ripen the cellulose acetate.

After the ripening step was completed, a magnesium acetate aqueoussolution was added to the ripened solution with stirring. The resultingsolution was added to 10% by weight acetic acid aqueous solution to givea precipitate. The formed precipitate was divided into two parts, andeach part was collected by filtration, and then washed with flowing warmand pure water. After draining, the wet cellulose acetate was collectedin Examples 3 and 4.

(Analysis for Cellulose Acetate)

Regarding the cellulose acetate produced for Examples 3 and 4, thedegrees of substitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS),and the degree of polymerization thereof were measured. The degrees ofsubstitution were measured in accordance with Tezuka, Carbohydr. Res.273, 83 (1995). First, dissociated (or free) hydroxyl groups in thesample (cellulose acetate) are changed into propionate esters withpropionic acid anhydride in pyridine. Thus obtained sample was thendissolved in deuteriotrichloromethane, and a ¹³C-spectrum was measured.The carbonyl carbons in the acetyls at 2-, 3- and 6-positions givesignals in the order from higher magnetic field in the range of 169 ppmto 171 ppm. The carbonyl carbons in the propionate esters at 2-, 3- and6-positions give signals in the order from higher magnetic field in therange of 172 ppm to 174 ppm. According to the obtained signals, theratio between acetyl and propionyl at 2-, 3- or 6-position wasdetermined to obtain the distribution of acetyls in the sample celluloseacetate. In accordance with this analytical method, the values of 6DS,2DS and 3DS were 0.901, 0.945 and 0.941, respectively. The degree ofpolymerization (average degree of polymerization) was 284. Concerningthese numeric values, there was no significant difference betweenExamples 3 and 4.

[Post-Treatment]

This cellulose acetate was subjected to the post-treatment in the sameway as in Examples 1, 2 and 5 to 9 and Comparative Examples 1 to 3. Thatis, in Examples 3 and 4, these cellulose acetates were dipped fortreatment with calcium hydroxide aqueous solutions each having aconcentration of 5 to 30 ppm different from each other. Then, each ofthe treated matter was filtrated off and drained, and dried withhot-air. Further, the heat resistance under wet condition and thereleasability of the film by the casting process were evaluated. Theseresults are shown in Table 2.

As apparent from Table 2, the cellulose esters of Examples 3 and 4 areexcellent in the heat resistance under wet condition. Further, thereleasability is also favorable. TABLE 2 Heat Amount of resistancesulfuric under wet Test Ca Mg acid Ca condition Ca Mg Examples (ppm)(ppm) (ppm) SO₄ (%) (10⁻⁷ mol) (10⁻⁷ mol) Releasability Ex. 3 71.5 3.0250 0.69 0.049 17.8 1.2 A Ex. 4 66.1 3.0 200 0.79 0.051 16.5 1.2 A

1. A cellulose ester fulfilling the following formula:0.5<(B)/(A)<1.2 wherein (A) is the total amount (in terms of molaramount) of residual sulfuric acid in 1 gram of said cellulose ester, and(B) is the total amount (in terms of molar amount) of calcium containedin 1 gram of said cellulose ester, and the cellulose ester is (i) acellulose acetate having an average degree of acetylation from 58 to62.5%, or (ii) a mixed fatty acid ester of cellulose.
 2. A celluloseester according to claim 1, which fulfills the following formula:0.75<(B)/(A)<1.0 wherein (A) is the total amount (in terms of molaramount) of residual sulfuric acid in 1 gram of said cellulose ester, and(B) is the total amount (in terms of molar amount) of calcium containedin 1 gram of said cellulose ester.
 3. A cellulose ester according toclaim 1, wherein the total amount (A) of residual sulfuric acid in 1gram of said cellulose ester is from 1×10⁻⁷ to 500×10⁻⁷ mol.
 4. Acellulose ester according to claim 1, wherein the total amount (B) (interms of molar amount) of calcium contained in 1 gram of said celluloseester fulfills the following formula.5×10⁻⁷<(B)<20×10⁻⁷
 5. (canceled)
 6. (canceled)
 7. A cellulose esteraccording to claim 1, which is the cellulose acetate (i), wherein thedegrees of substitution at 2- and 3-positions are not less than 1.70 andnot more than 1.95, and the degree of substitution at 6-position is notless than 0.88.
 8. A cellulose ester according to claim 1, which is thecellulose acetate (i), wherein the degrees of substitution at 2- and3-positions are not less than 1.84 and not more than 1.92, and thedegree of substitution at 6-position is not less than 0.89 and not morethan 0.92.
 9. An optical film comprising a cellulose ester recited inclaim
 1. 10. An optical film according to claim 9, which is a polarizerprotective film, a retarder film, a scattering film, or a widen viewingangles film.
 11. A process for producing a cellulose ester recited inclaim 1, which comprises acylating a cellulose in the presence ofsulfuric acid catalyst, and hydrolyzing the acylated cellulose to obtainthe cellulose ester; and further includes a step for neutralizing saidsulfuric acid at least in part with a neutralizing agent, and a step foradding a calcium component thereto.
 12. A process according to claim 11,which comprises acetylating the cellulose with an acetylating agent inthe presence of sulfuric acid catalyst, hydrolyzing the acetylatedcellulose, and adding the calcium component thereto to obtain thecellulose ester, wherein the molar ratio (B)/(A) determined from (A) thetotal amount (in terms of molar amount) of residual sulfuric acid in 1gram of the cellulose ester and (B) the total amount (in terms of molaramount) of calcium contained in 1 gram of said cellulose ester isadjusted into the range of the following formula:0.5<(B)/(A)<1.2 by addition of calcium hydroxide as the calciumcomponent, and the addition of calcium hydroxide is carried out aftersaid sulfuric acid is neutralized at least in part with a neutralizingagent containing a magnesium component at (i) the time when theacetylating step is finished and the hydrolyzing step is not startedyet, or (ii) the time when the hydrolyzing step is finished and theadding step of the calcium component is not finished yet.
 13. (canceled)